Brownian relaxation of a magnetic particle system with thermally blocked magnetic single domains (in the particles) can be detected by sweeping the frequency of the measuring field and measure the magnetic response, i.e. the real and imaginary part of the magnetic response, see for example WO 03/019188, by the same applicant and incorporated herein by reference. At the characteristic Brownian relaxation frequency the real part of the magnetic response decreases and the imaginary part exhibit a maximum. From the frequency dependent magnetic response the Brownian relaxation frequency can be determined. This method may be sensitive since it is possible to use the sensitivity of the lock-in amplifier technique in order to detect the magnetic response from the detection coil system placed in (the centre of) an excitation coil. The sensitivity of this technique can be further increased by tuning the resonance of the detection coil by an external capacitor in such way that the measurement at each excitation frequency is made at the peak value of the response of the detection coils. However, this technique requires that the frequency is swept over the Brownian relaxation frequency peak. This technique is described below.
Moreover, the detection in the frequency domain is usually relatively slow and requires changes of the sample position from one detection coil to another.